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Wake turbulence

This picture from a NASA study on wingtip vortices qualitatively illustrates the wake turbulence.

Wake turbulence is a disturbance in the atmosphere that forms behind an aircraft as it passes through the air. It includes various components, the most important of which are wingtip vortices and jetwash. Jetwash refers simply to the rapidly moving gases expelled from a jet engine; it is extremely turbulent, but of short duration. Wingtip vortices, on the other hand, are much more stable and can remain in the air for up to three minutes after the passage of an aircraft. It is therefore not true turbulence in the aerodynamic sense, as this would be chaotic. Instead, it refers to the similarity to atmospheric turbulence as experienced by an aircraft flying through this region of disturbed air.

Wingtip vortices occur when a wing is generating lift. Air from below the wing is drawn around the wingtip into the region above the wing by the lower pressure above the wing, causing a vortex to trail from each wingtip. The strength of wingtip vortices is determined primarily by the weight and airspeed of the aircraft.[1] Wingtip vortices make up the primary and most dangerous component of wake turbulence.

Wake turbulence is especially hazardous in the region behind an aircraft in the takeoff or landing phases of flight. During take-off and landing, aircraft operate at high angle of attack. This flight attitude maximizes the formation of strong vortices. In the vicinity of an airport there can be multiple aircraft, all operating at low speed and low altitude, and this provides extra risk of wake turbulence with reduced height from which to recover from any upset.

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At altitude, vortices sink at a rate of 90 to 150 metres per minute and stabilize about 150 to 270 metres below the flight level of the generating aircraft. For this reason, aircraft operating greater than 600 metres above the terrain are considered to be at less risk.[2]

Helicopters also produce wake turbulence. Helicopter wakes may be of significantly greater strength than those from a fixed wing aircraft of the same weight. The strongest wake can occur when the helicopter is operating at lower speeds (20 to 50 knots). Some mid-size or executive class helicopters produce wake as strong as that of heavier helicopters. This is because two-blade main rotor systems, typical of lighter helicopters, produce a stronger wake than rotor systems with more blades. The strong rotor wake of the Bell Boeing V-22 Ospreytiltrotor can extend beyond the description in the manual, which contributed to a crash.[3]

During takeoff and landing, an aircraft's wake sinks toward the ground and moves laterally away from the runway when the wind is calm. A 3 to 5 knot crosswind will tend to keep the upwind side of the wake in the runway area and may cause the downwind side to drift toward another runway. Since the wingtip vortices exist at the outer edge of an airplane's wake, this can be dangerous.

Even though the resolution to add the "Super" category is still under consideration, both the FAA (see below) and EUROCONTROL have already implemented guidelines concerning the Airbus A380.

There are a number of separation criteria for take-off, landing and en-route phases of flight based upon these categories. Air Traffic Controllers will sequence aircraft making instrument approaches with regard to these minima. Aircraft making a visual approach are advised of the relevant recommended spacing and are expected to maintain their own separation.

The FAA does not use the ICAO categories for wake turbulence separation, instead using the following:[6]

Additionally, the Boeing 757, which by MTOW and ICAO rules is considered a large aircraft, has the wake separation rules of heavy aircraft applied to it for purposes of separation because of a number of incidents where smaller aircraft lost control (with some crashing) while following closely behind a 757.[8]

Common minima are:

Take-off

An aircraft of a lower wake vortex category must not be allowed to take off less than two minutes behind an aircraft of a higher wake vortex category. If the following aircraft does not start its take off roll from the same point as the preceding aircraft, this is increased to three minutes. To put this more generally, an aircraft is usually safer if it is airborne before the rotation point of the airplane that took off before it. However, care must be taken to stay upwind (or otherwise away) from any vortices that were generated by the previous aircraft.[9]

In 2012 the FAA authorized Memphis, Tennessee air traffic controllers to begin applying revised criteria,[11] which retained the previous weight categories but also addressed differences in approach speeds and wing configuration. This resulted in six categories of aircraft, and the revised spacing allowed among these categories was soon shown to increase airport capacity.[12] The gain in capacity at Memphis was significant, with an FAA-estimated increase in capacity of 15%, and average taxi time for FedEx (Memphis' largest carrier), with about 500 operations per day in 2012) aircraft was cut by three minutes.[13]

FAA has continued development of RECAT. The FAA's overall plan is to slowly phase in more complex factors to allow reduced wake separation, in order to increase capacity. RECAT Phase I (first demonstrated in Memphis), introduces 6 static wake turbulence categories to replace the traditional weight classes. The FAA used maximum takeoff weight, maximum landing weight, wingspan, and approach speed in Phase I in order to more accurately represent the wake severity of a generating aircraft, as well as the vulnerability of trailing aircraft to a potential wake encounter. This analysis enables the development of more efficient wake turbulence separation minima than those specified in the baseline operational rules specified in FAA Order JO 7110.65. As of April 2016, RECAT Phase I has been implemented at 10 TRACON and 17 airport locations.

RECAT Phase II is a continuation of the RECAT program that focuses on a larger variety of aircraft (123 ICAO type designators that make up more than 99% of US air traffic movements based on 32 US airports), as opposed to the 61 aircraft comprising 85% of operations from 5 US and 3 European airports that were used in RECAT Phase I. The fundamental underlying wake separations in RECAT Phase II are not defined per wake turbulence category, but actual individual pairs of make-model-series aircraft types (e.g. Boeing B747-400 leading Airbus A321). In the US, automation does not yet exist to allow air traffic controllers to utilize this pairwise separation matrix. Instead, RECAT Phase II takes advantage of the underlying matrix to redefine the RECAT Phase I-type categories (i.e. Categories A – F, with an additional Category G) for individual TRACONs. This allows further efficiency gains over RECAT I because it takes the fleet mix – which aircraft fly most often – into account for each site, rather than doing a global optimization for the US national airspace system as a whole.[14] RECAT Phase II went operational on August 3, 2016 at Southern California TRACON and associated towers.[15]

With the largest global wake database,[16]EUROCONTROL has developed advanced wake metrics to set up the European six category wake turbulence separation minima, RECAT-EU, as an alternative to the long established ICAO PANS-ATM categories, in order to safely support an increase in runway throughput at airports in Europe. RECAT-EU also integrates a Super Heavy category for the Airbus A380 bringing runway capacity benefits of up to 8% or more during peak traffic periods. As part of the wake turbulence recategorisation separation review, SESAR partners EUROCONTROL and NATS have developed RECAT-EU from the long understood concept of Time based separation (TBS).[17]

Following approval by the European Aviation Safety Agency (EASA), RECAT-EU is to be initially deployed at Paris Charles de Gaulle airport by end-2015.[18][19][20]

Eurocontrol plans to move beyond RECAT EU onto a more granular separation matrix whereby precise separations for each of an initial 115 common commercial aircraft are defined by model in a 'Pair Wise Separation' (PWS) system.

These separation matrices known as Recat 2 and Recat 3 are to be deployed in European airports towards 2020 and 2022 respectively.[21]

Incident data shows that the greatest potential for a wake vortex incident occurs when a light aircraft is turning from base to final behind a heavy aircraft flying a straight-in approach. Light aircraft pilots must use extreme caution and intercept their final approach path above or well behind the heavier aircraft's path. When a visual approach following a preceding aircraft is issued and accepted, the pilot is required to establish a safe landing interval behind the aircraft he was instructed to follow. The pilot is responsible for wake turbulence separation. Pilots must not decrease the separation that existed when the visual approach was issued unless they can remain on or above the flight path of the preceding aircraft. Having a higher approach path and touching down further along the runway than the previous aircraft will help avoid wake turbulence.

Finally it should be noted that glider pilots routinely practice flying in wingtip vortices when they do a maneuver called "boxing the wake." This involves descending from the higher to lower position behind a tow plane. This is followed by making a rectangular figure by holding the glider at high and low points away from the towing plane before coming back up through the vortices. (For safety this is not done below 1500 feet above the ground, and usually with an instructor present.) Given the relatively slow speeds and lightness of both aircraft the procedure is safe but does instill a sense of how strong and where the turbulence is located.[22]

Any uncommanded aircraft movements (such as wing rocking) may be caused by wake. This is why maintaining situational awareness is critical. Ordinary turbulence is not unusual, particularly in the approach phase. A pilot who suspects wake turbulence is affecting his or her aircraft should get away from the wake, execute a missed approach or go-around and be prepared for a stronger wake encounter. The onset of wake can be insidious and even surprisingly gentle. There have been serious accidents where pilots have attempted to salvage a landing after encountering moderate wake only to encounter severe wake turbulence that they were unable to overcome. Pilots should not depend on any aerodynamic warning, but if the onset of wake is occurring, immediate evasive action is vital.

8 June 1966 – an XB-70 collided with an F-104. Though the true cause of the collision is unknown, it is believed that due to the XB-70 being designed to have an enhanced wake turbulence to increase lift, the F-104 moved too close, therefore getting caught in the vortex and colliding with the wing (see main article).

15 December 1993 – a chartered aircraft with five people on board, including In-N-Out Burger's president, Rich Snyder, crashed several miles before John Wayne Airport. The aircraft was following a Boeing 757 for landing, became caught in its wake turbulence, rolled into a deep descent and crashed. As a result of this and other incidents involving aircraft following behind a Boeing 757, the FAA now employs the separation rules of heavy aircraft for the Boeing 757.

20 September 1999 – A JAS 39A Gripen from Airwing F 7 Såtenäs crashed into Lake Vänern in Sweden during an air combat maneuvering exercise. After passing through the wake vortex of the other aircraft, the Gripen abruptly changed course, and pilot Capt. Rickard Mattsson, got a highest-severity warning from the ground-collision warning system. He ejected from the aircraft, and landed safely by parachute in the lake.

8 July 2008 – A US Air Force PC-12 trainer crashed at Hurlburt Field, Fla., because the pilot tried to land too closely behind a larger AC-130U Spooky gunship and got caught in the gunship’s wake turbulence. Air Force rules require at least a two-minute separation between slow-moving heavy planes like the AC-130U and small, light planes, but the PC-12 trailed the gunship by about 40 seconds. As the PC-12 hit the wake turbulence, it suddenly rolled to the left and began to turn upside down. The instructor pilot stopped the roll, but before he could get the plane upright, the left wing struck the ground, sending the plane skidding 669 feet across a field before stopping on a paved overrun.[24]

3 November 2008 – Wake turbulence of an Airbus A380-800 causing temporary loss of control to a Saab 340 on approach to a parallel runway during high crosswind conditions.[25]

4 November 2008 – In the infamous 2008 Mexico City plane crash, a LearJet 45 XC-VMC carrying Mexican Interior Secretary Juan Camilo Mouriño, crashed near Paseo de la Reforma Avenue before turning for final approach to runway 05R at Mexico City International Airport. The airplane was flying behind a 767-300 and above a heavy helicopter. The pilots were not told about the type of plane that was approaching before them, nor did they reduce to minimum approach speed.[citation needed] (This has been confirmed as the official stance by the Mexican Government as stated by Luiz Tellez, the Secretary of Communications of Mexico.)[citation needed]

On 9 September 2012, a Robin DR 400 crashed after rolling 90 degrees in a wake turbulence induced by the preceding Antonov AN-2, three killed, one severely injured.[26][27]

On 28 March 2014, Indian Air Force C-130J-30 KC-3803 crashed near Gwalior, India, killing all 5 personnel aboard.[28][29][30] The aircraft was conducting low level penetration training by flying at around 300 ft when it ran into wake turbulence from another aircraft in the formation, which caused it to crash.[31]

On 7 January 2017, a private Bombardier Challenger 604 rolled three times in midair and dropped 10,000 feet after encountering wake turbulence when it passed 1,000 feet under an Airbus A380 over the Arabian Sea. Several passengers were injured, one seriously. Due to the G-forces experienced, the plane was damaged beyond repair and was consequently written off.[32]

Wake turbulence can be measured using several techniques. Currently, ICAO recognizes 2 methods of measurement, sound tomography, and a high-resolution technique is Dopplerlidar, a solution now commercially available. Techniques using optics can use the effect of turbulence on refractive index (optical turbulence) to measure the distortion of light that passes through the turbulent area and indicate the strength of that turbulence.

Wake turbulence can occasionally, under the right conditions, be heard by ground observers.[33] On a still day, the wake turbulence from heavy jets on landing approach can be heard as a dull roar or whistle. This is the strong core of the vortex. If the aircraft produces a weaker vortex, the breakup will sound like tearing a piece of paper. Often, it is first noticed some seconds after the direct noise of the passing aircraft has diminished. The sound then gets louder. Nevertheless, being highly directional, wake turbulence sound is easily perceived as originating a considerable distance behind the aircraft, its apparent source moving across the sky just as the aircraft did. It can persist for 30 seconds or more, continually changing timbre, sometimes with swishing and cracking notes, until it finally dies away.

In the 1986 film Top Gun, Lieutenant Pete "Maverick" Mitchell, played by Tom Cruise, suffers two flameouts caused by passing through the jetwash of another aircraft, piloted by fellow aviator Tom "Ice Man" Kazansky (played by Val Kilmer). As a result, he is put into an unrecoverable spin and is forced to eject killing his RIO Nick "Goose" Bradshaw. In a subsequent incident, he is caught in an enemy fighter's jetwash, but manages to recover safely.

In the movie Pushing Tin, air traffic controllers stand just off the threshold of a runway while an aircraft lands, in order to experience wake turbulence firsthand. However, the film dramatically exaggerates the effect of turbulence on persons standing on the ground, showing the protagonists being blown about by the passing aircraft. In reality, the turbulence behind and below a landing aircraft is too gentle to knock over a person standing on the ground. (In contrast, jet blast from an aircraft taking off can be extremely dangerous to people standing behind the aircraft.)